1. Field of the Invention
The present invention relates to solid-state light-emitting devices and, more particular, semiconductor laser devices that are preferably used as solid-state light sources in an optical data-processing apparatus, an optical measuring apparatus, or the like.
2. Description of the Related Art
In recent years, solid-state light-emitting devices are necessarily used in processing optical data, optical measuring of physical values, and similar technical fields. A semiconductor laser, for example, is often used as a light source for writing data onto, or reading data from, a disk-shaped optical data storage media. In such cases, it should be required that the semiconductor laser absolutely has excellent basic beam characteristics. To meet the technical requirement, the semiconductor laser must be satisfied with the following specifications: it exhibits a stable fundamental transverse mode oscillation throughout its high power output range, while the beam astigmatism is kept little.
With the presently available manufacturing technology, it is by no means easy to manufacture laser diodes with fundamental transverse-mode oscillation, for the following reasons. Whether or not a semiconductor laser device can perform fundamental transverse-mode oscillation greatly depends on the structural dimensions, such as shape, size, or the like, (in particular, the width W) of the stripe-shaped light-guiding groove (ridge waveguide) portion of the laser device. The ridge waveguide portion can be formed by existing etching process to have a sufficiently precise width W if the mask used in the etching process has accurate dimensions. However, the thickness of the cladding layer, which is located just beneath the waveguide portion, cannot be satisfactorily controlled unless the etching conditions are minutely and precisely adjusted. To adjust these conditions precisely in accordance with varying factors concerning the manufacture of the semiconductor laser device, it is necessary for the manufacturer to employ high technology which has not been put to general use. Introducing such highly advanced, special technology would be a great risk to the semiconductor manufacturers, from a commercial point of view.
"Extended Abstract of 17th Conference on Solid State Devices an Materials," Tokyo (1985), pp. 67-70, discloses and m-ECO type semiconductor laser. This device exhibits fundamental transverse-mode oscillation within its optical output range. Reportedly, the device is excellent in beam characteristics such as beam astigmatism characteristic. The effective refractive index within the stripe-shaped, grooved portion of this laser device is different from that outside the grooved portion, so that the device can perform stable transverse-mode oscillation. In addition, since the current-blocking layer of the laser device is made of gallium-aluminum-arsenide (GaAlAs), the optical loss outside the grooved portion is small, resulting in reduction of astigmatism.
The semiconductor laser disclosed in the above-identified document, is disadvantageous, however, in that the crystalline quality of the interface of the stripe-shaped, grooved portion is deteriorated. More specifically, the laser has been made by forming, among other components, an optical guide layer on the stripe-shaped etched groove, a cladding layer on the optical guide layer, and a contact layer on the cladding layer, by means of crystal growth. Hence, semiconductor crystals grow further on the aluminum-containing layer included in the grooved portion, as an electric current flows through the grooved portion while the laser is emitting a beam. The more aluminum the cladding layer contains, the more the crystalline quality is deteriorated. Therefore, the m-ECO type semiconductor laser cannot be modified into indium-gallium-aluminum-phosphide (InGaAlP) semiconductor lasers.
Another document, "Extended Abstract of 17th Conference on Solid State Devices and Materials," Tokyo (1986), pp. 153-156, discloses a ridge-stripe type InGaAlP laser which has been designed to solve the problem inherent in the m-ECO type semiconductor laser. Since this InGaAlP laser has been made by a method which includes no steps of re-growing crystals on the aluminum-containing semiconductor layer, the interface of the stripe-shaped, grooved portion has good crystalle quality. However, this laser will have a great optical loss if the stripe-shaped ridge section has a small width W in order to improve the mode stability in high-power oscillating operation. Inevitably, the astigmatism characteristic of the laser will be deteriorated, though the laser performs a stable transverse mode oscillation. Obviously, the laser can neither perform a stable transverse-mode oscillation nor have good beam characteristic, if designed to emit a high optical output.
Either of the conventional laser devices, described above, has a transverse-mode characteristic which greatly depends on the width W of the stripe-shaped portion. Hence, the stripe-shaped portion must have a desired width W. To form the a stripe-shaped portion having the desired width W, the manufacturer of the laser device need to employ high technology which has not been put to general use. Since such high technology is too expensive to utilize, it remains practically difficult to manufacture a semiconductor laser device which has a good beam characteristic and performs a stable transverse-mode oscillation, to emit a high optical output.